About amperage ratings for power tools??

[steve-u]

I was wondering if there is any standards that power tool manufactures use to rate power tools amperage? The reason I ask is I got one of those Kill A Watt devices that can measure such primarily to measure power used by all the various computers in the house.

http://www.the-gadgeteer.com/review/kill_a_watt_electric_usage_monitor_review

So I figured I would try it out on my power tools to see what the results were. Here were my findings on amps used on various tools with no load after about 15 seconds from startup along with "rated" amps.

Pro Tech 10 CMS - 5.5 rated 15
Ryobi BT3100 TS - 6.8 rated 15
Craftsman RAS - 9.8 rated 11
Hitachi 12 CMS - 8.2 rated 15
Milwaukee drill - 2.5 rated 5.5
Rigid 2.5 HP vac - 7.2 rated 7.3

Interestingly my Craftsman RAS used the most idle amps and was rated lowest amperage of my saws I tested. I tried the RAS when crosscutting
a 1X6 board and it went up .2 amps. I also have to wonder why my two
CMS that are both rated at 15 amps had such a large difference in idle amperage. I know the Hitachi has a larger blade but I can't imagine that
would account for the 2.7 amps difference. I watched as I used a hole saw
for drilling with my Milwaukee Hole Shooter drill and the amperage draw went up to 5.0 amps which is close to it's rated amps. The Rigid 2.5 HP vac was right at rated amps and if put my hand over the end of the tube the amperage actually dropped to 5.5 amps.

Even though the Craftsman RAS is rated the lowest in amps of my saws my test shows it used the most idle amps by far and my experience is that it is easily my most powerful saw despite the amp ratings. It is however 25 years old and maybe back then they rated amps more realistically or to some standard no longer followed? I guess my point is that after reviewing results I don't really believe in the amps rating of various power tools is a reliable indicator of the performance of the tool. --- Steve

[cwsmith]

Steve,

In my opinion, "Amps" have become almost a "marketing" ploy to indicate "power". We all know that most Americans prefer "more power" in almost everything they do, whether it be their vacuum cleaners, their tools, or their vehicles. Heck even entertainment equipment touts "more watts" than is actually realistic. The "Power" thing has certainly become an ever growing theme for marketing anything... even toothbrushs!

So while I'm sure there is some formula that is used to calculate the amps or the HP, it is most likely exagerated as much as possible. Your test is perhaps illustrative of exactly how much exageration has occured over the last 20 years or so. But then again, who really knows how much "efficiency" has been engineered into these things over the years; and, how much the ratings may now take into consideration actual or estimated load capability that the motor may be capable of. Still, I suspect that there's a bit of exageration going on.

CWS

[steve-u]

That is prety much what I suspect. In 1982 I bought a hafler DH200 stereo amplifier because I wanted 100 watts per channel in my stereo system. It weighs about 30 pounds and has massive capacitors and heatsinks in it. I still use it today. Today you can see in Walmart that they sell car stereos or boom boxes rated at 100 watts per channel for $39. Of course in this case the difference is that my hafler is rated at RMS into 8 ohms at very low distortion. I have no idea how they justify today's watt rating. --- Steve

[LCHIEN]

I can explain a few things.

First of all, lets talk vacs and Dust collectors.
These things draw maximum amps when moving the most air. They make a log of noise and somehow you think they are working harder whent he air flow is blocked, but that is actually the lowest amperage mode. So what yousee on the vac is normal, running at peak amperes when the hoses are open and lower current when you block the hose completely.

For all other motors that cut stuff or grind stuff, they draw a few amps when just spinning at low loads (and this includes friction, keeping the balde rotating, wind resistance, motor inefficiency, etc), maybe some lights, etc. But they will draw nowhere near the nameplate amps. I recently measuresd some with my Kill-a-watt meter and a Fluke 87 DVM and got similar values, very roughly 1/2 the nameplate amps on the average. Most motors and hence the machines they power are rated according to a formula that rates the motor slip (for induction motors) and hence torque as a percentage of the nominal synchronous rate, where some percent slippage is fully loaded (I think about 90% or a drop from 3600 to 3240 rpm) and the current where that occurs is the nameplate rating. Assuming the motor is also properly designed and applied that amperage is also less than the current carrying capacity of its internal windings.

If you apply additional torque load to the motor then the current will increase from the idle current (as you call it) in proportion to the load. So a 1x6 fed slowly is a small load increase, fed faster its a larger load and a 4x4 fed fast is a big load. If you cut a thick piece at a high feed rate you will probably at some point approach the maximum rated amperage. Somewhere past that point, your motor will bog down and overheat and if kept this way for a while will result in the burnout of the motor when the winding insulations break down and short out. Once the speed slip is below 90% of full speed then the torque falls rapidly and the motors windings are saturated efficiency falls off fast.

Back to the vac motors, the reason that it runs at full current when the air path is clear is that the impeller size and the motor have been chosen together to work that way. A larger impeller with the same motor would burn up if left running like that. A smaller impeller and the same motor would waste motor power because it could never operate near the 100 load capability of the motor and the designer wasted money on too big a motor.

So the answer is that there are logical ways in which these things are measured and except for the Vac, you have not measured these things at anywhere near maximum torque. In fact the way to measure maximum torque given the nameplate rating is to load the machine up until you hit the amperage given and then load no more. as i said before, you load them up by cutting thicker, harder woods at greater feed rates.

Now there are some vendors (Craftsman, notably) and vac and air compressor manufacturers that quote some form of developed horsepower which is not a good way to measure available power. Basically this is based on the short circuit current when the motor is stalled and is basically just the DC reisistance of the windings and is not a good point of comparison for anything. They like to use it because it is always higher than HP derived from the nameplate amps.

[Jim Frye]

What I know about electricity could kill me

Loring: Great post! It brings up a question for me. I'll use my old BT3000 as the example. My saw is an early model with the 13 amp. motor. I have it plugged into a 20 amp. circuit wired with 12 ga. wire and the power cord is also 12 ga. wire. If I load this saw up enough, it will draw enough current to trip the breaker. I don't own a good meter (30 year old $6 Radio Shack VOM), so I have no idea what amps. this thing is pulling when the breaker trips. My impression is this motor is "overreaching" its 13 amp. rating. Is the rise past 13 amps. so fast that it is not appearant until the breaker trips? I mean, I don't recognize any bogging or slow down until right before I lose power.

[Crash2510]

Starting amps for these motors are usually much higher which means that these motors have the potential peak amperage higher than what is listed thats why they are usually wired with a guage heavier than required than normal load amperage another reason for a heavier gauge is heat. Also sometimes a hot motor can cause a breaker to trip If they don't already have overload protection.

Jim is your bt on a circuit with any other things in your house

also how often does this happen

[Crash2510]

One more thing if I can't help you maybe this will clear things up

A motor may trip your circuit breaker on time-overcurrent (the
heaters) even if the motor nameplate current rating appears to be
within the breaker rating. This can happen if you continuously
overload the motor; motor current will then be several times the
nameplate rating. There may be other signs of this. The motor may
become extremely hot (spit sizzles on the casing). This is General
Electric's way of telling you to slow down.

http://www.faqs.org/ftp/faqs/woodworking/motors

[LCHIEN]

Loring: Great post! It brings up a question for me. I'll use my old BT3000 as the example. My saw is an early model with the 13 amp. motor. I have it plugged into a 20 amp. circuit wired with 12 ga. wire and the power cord is also 12 ga. wire. If I load this saw up enough, it will draw enough current to trip the breaker. I don't own a good meter (30 year old $6 Radio Shack VOM), so I have no idea what amps. this thing is pulling when the breaker trips. My impression is this motor is "overreaching" its 13 amp. rating. Is the rise past 13 amps. so fast that it is not appearant until the breaker trips? I mean, I don't recognize any bogging or slow down until right before I lose power.

Jim, for one thing, circuit breakers are popularly considered to be cliff devices, where you go over the cliff at 20A if its a 20A breaker. In reality, like all things manufactured, they have tolerances, might be 5 or 10%. But even more than that, they have overload vs time characteristics. Let me list the characteristics of a hypothetical CB:
Suppose its accurately 20A rated.
At 21 A it will run for 1 hour before tripping
At 22A, it will run for 10 minutes before tripping
at 25 amps it will run for 1 minute before tripping
at 30A it will run for 5 seconds before tripping
at 40A, it will trip in less than .5 second

So I will tell you this, that your saw is running well over 13 Amps, or, your breaker has gotten weak from old age and stress.

It may be that your saw is running 15 or more amps - more than 20 is somehow questionable in my mind.

Universal motors are different than induction motors. The torque loss is not as sudden and dramatic, IIRC. The speed decreases steadily with torque. They can draw excess current to add more torque unlike induction motors once they pass a certain slip rate they lose it.

Now a 13Amp motor is only where they rated it. There are always saftey factors in engineering, allowing for operation at greater temperatures, lower line voltages etc. For example the 13 A may apply at 120F and 110V input, you will get a lot more power when you have 122V and its a 60degree F balmy day and you saw will be forgiving.

If you have $25 (with shipping) to spare, get one of those kill-a-watt meters from eBay, they plug inthe wall and appliances or tools plug into them. They have voltage, amps, power factor, power, VA reactive power and KWH and elapsed hours displays. I wish it had a peak hold function, its a little dangerous to be feeding wood and watching a meter! Seems reasonably accurate (+/-5%) to this engineer.

Back to your question, its quite possible that your saw is drawing more than 13Amps, almost certainly before it trips the breaker.

[Jim Frye]

Many things answered

"Jim is your bt on a circuit with any other things in your house"

The BT is on a dedicated circuit when it is operating. This circuit was put in just for the saw and any other high amp. tools (planer, plunge router, BT(s)). I also replaced the breaker a couple of years ago just to make sure wear and tear hadn't caused an unsafe condition.

"also how often does this happen"

The BT trips out only on really heavy loads, like resawing 3" or greater hard woods.

Loring's explanation is right on. As a further example of amperage pull, I'll cite my AP1300 planer. On the 20 amp. shop circuit, it has never tripped the breaker. If I take it outside and run it on the 15 amp. garage circuit with it's long run from the breaker panel and 14 ga. wire, it will trip the breaker after about 16 feet of continuous work. I found this out last summer trying to plane 15' & 16' red oak boards. I had to do a board and take a break before making the next pass.

[JimD]

Jim

Tripping a 20A breaker with a 13A saw is curious. I have my newer 15A BT3100 on a 20 amp circuit and have cut 3 1/2 inches in oak several times with no breaker trips. I have never tripped the breaker with any of my tools. But you've replaced the breaker. Curious. I doubt this is an issue but I would be sure to use a 12 guage and as short as possible an extension cord.

I will also compliment Loring on his explanation.

Jim

[steve-u]

Thanks for the great info Loring. As i said I was able to put the Milwaukee drill under load and it indeed did come close to rated amps. Unfortuneatly being the only one in my basement workshop at the time I was not going to attempt heavy load tests on my saws while trying to watch the Kill A Watt meter at the same time.

I still think there are some major inconsistencies such as the difference in non load current drawn between my Pro Tech and Hitachi miter saws for instance - 5.5 compared to 8.2 though they are both rated at 15 amps. When I get a chance I need to get some challenging lumber to cut and a helper to gather more data. --- Steve

[cwsmith]

Loring,

Thanks for the great explanation. One of the things that I really love about this forum is the level of expertice and the willingness of people like you to provide such educational information.

Very much appreciated,

CWS

[TheRic]

Loring - Great posts!!!

Jim - Did the old breaker trip out like the new one, or only the new one does? It's possible the new breaker was bad / weak from the start. Try flipping the 20 amp breaker with a different 20 amp breaker. I have also seen moving the breaker to a different spot in the panel box make a difference. Make sure all the connections are clean, wires screwed down tight in panel box, receptacle, plugs, etc. Is the receptacle a true 20 amp receptacle? I personally don’t trust the quick connections on receptacles for heavy loads, I prefer to use the screws on circuits that I know will be used heavily. Does any of the wires / plugs get hot when this happens? If so, replace it / them.

[linear]

I'd like to nominate this thread for the "best of" forum.

[Jim Frye]

Been There, Done That

Loring - Great posts!!!

Jim - Did the old breaker trip out like the new one, or only the new one does? It's possible the new breaker was bad / weak from the start. Try flipping the 20 amp breaker with a different 20 amp breaker. I have also seen moving the breaker to a different spot in the panel box make a difference. Make sure all the connections are clean, wires screwed down tight in panel box, receptacle, plugs, etc. Is the receptacle a true 20 amp receptacle? I personally don’t trust the quick connections on receptacles for heavy loads, I prefer to use the screws on circuits that I know will be used heavily. Does any of the wires / plugs get hot when this happens? If so, replace it / them.

The new breaker does not trip out nearly as often as the 15 year old one did.

Don't have the time to rearrange the circuits in the box.

Connections are clean and tight.
I replaced the receptales some time ago as the ones the previous owner put in the shop were real cheapos and were actually disassembling when you would put a plug in. I replaced them with the best I could find and rewired the entire circuit with 12 ga. wire at the same time. These outlets were installed under work benches, so it was an easy job. The wiring at the outlets is indeed done with the screws.

There is no detectable warming of any part of the circuit.

I run the saw and the other 15 amp. tools from the outlet nearest the breaker box (breaker to outlet distance is less than 5'). Power cord on the saw is less than 2' long and I am using a 10', 12ga. extension cord between the outlet and the saw. That's less than 17' of 12 ga. wiring.